Hormonal control of digestion

It has now been well established that gut hormones have a key role in controlling food intake and energy expenditure. The gut is the body’s largest hormone-producing organ, releasing more than 20 different peptide hormones, some of which target the brain to regulate appetite and influence the pleasure of eating.

Hormones – chemicals that ‘excite’

Hormones are chemical substances produced by specialised cells in the body known as endocrine cells. Once released, the hormone enters the bloodstream and is transported to its specific site of action.

Once the target cell has been stimulated, it responds in a variety of different ways such as producing new proteins, activating or deactivating enzymes within the cell or allowing the passage of substances into or out of the cell.

Classification of hormones

One way of classifying hormones is by their chemical structure. Hormones synthesised in the body from cholesterol are known as steroid hormones. The sex hormones oestrogen and testosterone are perhaps the best known examples of steroid hormones.

All other hormones are primarily made in the body from amino acids. These non-steroid hormones can be classified into 4 main groups:

Gut hormones

The gut hormones work in association with the gut’s extensive nervous system (enteric nervous system) and play a co-ordinating role in the control of appetite, the digestion of food, the regulation of energy balance and the maintenance of blood glucose levels. The gut continuously sends information to the brain regarding the quality and quantity of the food that is consumed.

The role that some of these hormones play is outlined below:

Ghrelin is produced in the stomach, and its function is to tell the brain that the body has to be fed. It increases appetite.

Gastrin is produced in the stomach when it is stretched. It stimulates the release of gastric juice rich in pepsin and hydrochloric acid.

Secretin is produced in the duodenum and has the effect of stimulating the pancreas to produce alkaline secretions as well as slowing the emptying of the stomach.

Cholecystokinin (CCK) is produced in the duodenum. It reduces appetite, slows down the emptying of the stomach and stimulates the release of bile from the gall bladder.

Peptide YY (PYY) is produced in the last part of the small intestine known as the ileum as well as parts of the large intestine. It plays a role in slowing down the passage of food along the gut, which increases the efficiency of digestion and nutrient absorption after meal.

Glucagon-like peptide 1 (GLP-1) is produced in the small intestine and colon and has multiple actions including inhibition of gastric emptying and appetite as well as the stimulation of insulin release.

Hormones and satiety research

Scientists from Plant & Food Research, such as Dr John Ingram, are currently investigating the ability of plant food components, such as polyphenols, to trick the intestinal sensing system into signalling to the brain a sense of fullness (satiation) and lack of appetite (satiety).

They are also investigating the ability of some indigestible carbohydrates (fibre) to serve as a food source for bacteria present in the large intestine. The bacteria break the fibre down to short-chain fatty acids such as butyrate. Increasing levels of butyrate in the large intestine promote the release of hormones that induce a feeling of satiety, which will result in a lower intake of food.

Balancing the energy budget

The regulation of food intake is complex. It involves gut hormones and the enteric nervous system as well as the brain. Although this regulatory system has evolved to its current successful form over the evolutionary history of humankind, it is now being placed under extra pressure in our modern western world.

Access to cheap, energy-rich food, mechanised transportation, non-manual labour and sedentary pastimes have placed the functioning of the food processing system for some people under considerable load. The mechanisms that control energy balance are offset, the result being body weight gain, eventually leading to obesity.

Most of the treatments for obesity target energy intake and expenditure and include dieting and physical exercise, surgery, drugs and many scientifically undocumented remedies. Most of these treatments are not very effective.

Perhaps a future outcome of Plant & Food Research’s gut hormone work will be specific plant foods or plant food extracts that can assist in the control of appetite and hence energy balance within the body.

Nature of science

Scientific research is frequently conducted across disciplines in a collaborative and co-ordinated way. Dr John Ingram’s research project, which centres on gut hormones and satiety, is a typical example of this.